Just curious, is that a weird cancelation stamp on the NCLs, or the way they were originally?
I'm going to take a stab at this question and say it's a weird/custom cancellation stamp.
Taking a closer look, I can see where some chips, the stamp has worn away, or has been cleaned off. If this is the case, it should be fairly easy to remove in a ultrasonic cleaner.
I would like to ask if the NCL set is leaded.